1. Field of the Invention
The present invention relates to a method of detecting a coordinate accurately by reading a position pointed on an input plane, and more particularly to a coordinate detecting method characterized by determining a general region at the time of detecting the coordinate.
2. Description of the Prior Art
A coordinate detecting technology for highly accurately reading a position pointed on a plane is exemplified by the applicant's Japanese Patent Application No. 60-117761. The coordinate detecting apparatus disclosed in the Japanese Patent Application sends scanning signals of the same phase simultaneously to at least two of a plurality of conductors embedded in a tablet in parallel relation, detects by a coordinate pointer signals produced by the scanning signals sent to the conductors, discriminates whether the polarity of the detected signal is inverted, then detects signal levels before and after the polarity is inverted, and calculates, based on the position in which the inversion of the polarity is discriminated and the signal level, the position pointed by the coordinate pointer.
FIG. 8 illustrates the principal construction of this prior art. Reference numeral 1 designates a switching circuit, 2 conductors embedded in an input plane in parallel relation, 3 a pickup for detecting, as an electrical signal, a magnetic field produced when a high-frequency current flows in the individual conductor, 4 an amplifier circuit, 5 a polarity discrimination circuit for discriminating the polarity of the signal detected and amplified by the pickup 3, 6-1, 6-2 sample/hold circuits, 7 an A/D converter, 8 an oscillator for supplying a high-frequency current to the individual conductor 2, 9 a driver, 10 a detector circuit, 11 an adder, 12 a control unit.
In FIG. 8, firstly, a general detection is made as to which conductor 2 the pickup 3 is near. Secondly, a highly accurate detection is made as to where the pickup 3 is disposed between the conductor 2 and a conductor adjacent thereto.
The first general detection will now be described in more detail. Desired data from the control unit 12 are transmitted to the switching circuit 1. Based on the data, the switching circuit 1 selects two alternate conductors 2, i.e. A and C, B and D, C and E, D and F, and so forth, to be scanned by the same high-frequency current generated by the oscillator 8 and the driver 9. The magnetic field developed by each two conductors 2 is detected in the form of an electrical signal by the pickup 3 and is amplified by the amplifier circuit 4. Then, the amplified signal is supplied to the polarity discrimination circuit 5 where a discrimination is made as to which pair of conductors shows a polarity inversion in the pickup signal. If the inversion of the polarity is discriminated, this message is transmitted to the control unit 12. Thus a general position of the pickup 3 between two conductors is detected.
The second, more accurate detection will now be described in more detail. Upon receipt of the message that the polarity inversion is detected, the control unit 12 supplies a hold pulse to a sample/hold circuit 6-1, at which time an input signal V.sub.2 amplified by the amplified circuit 4 detected from one of the two conductors detected by the detection circuit 10 is held and hence memorized. Then, the control unit 12 issues the message to the switching circuit 1 to apply the high-frequency current to the other conductor. Then a hold pulse is supplied to the sample/hold circuit 6-2, at which time an input signal V1, amplified by the amplifier circuit 4 and detected by the detection circuit 10 is held and hence memorized. The thus memorized signals V.sub.1 and V.sub.2 are supplied to the adder 11 to calculate a sum signal (V.sub.1 +V.sub.2). The sum signal (V.sub.1 +V.sub.2) is supplied to a reference voltage input terminal V.sub.REF of the A/D converter 7, and one of the signals V.sub.1, V.sub.2, for example, V.sub.2 is supplied to an input terminal V.sub.IN. The value of the following equation is calculated. ##EQU1##
By inputting the value of the denominator and the value of the numerator of the equation (1) to the reference voltage input terminal V.sub.REF of the A/D converter 7 and the input terminal V.sub.IN, respectively, it is possible to calculate the value of the equation (1). According to the equation (1), the value of X.sub.i is obtained, and the value of Y.sub.i is likewise obtained; then these values are transmitted to the control unit 12. Based on the input coordinate values (X.sub.i, Y.sub.i), the positional coordinates (X, Y) on the input plane are determined.
However, this prior art calculating method has the following problems. Although a calculation is made assuming that the polarity is inverted at a midpoint between the selected two conductors 2, this position of inversion would be shifted from the midpoint between the selected two conductors 2 due to the magnetic field generated from the common conductor through which the electric current is supplied to each of the two conductors 2, thus causing a significant calculation error. In an effort to avoid the influence of the common conductor, it has been proposed to encircle the selected two conductors by compensation loops. But, since the provision of this compensation loop does not completely avoid the calculation error, an interpolation is required in view of the shifting of the position of polarity inversion. Particularly in the case where scanning of the compensation loops is made one after another in order, this shifting causes an inaccurate scanning. More specific explanation is as follows.
FIG. 6 illustrates a detection voltage distribution in the absence of the positional shifting of the polarity inversion. In the region between 10 mm and 30 mm, a distribution of signals from the loops are shown designated as L10 L.sub.15, L.sub.20, L.sub.25, L.sub.30. Here, the distribution of the magnetic strength Hz corresponding to each loop L.sub.10, L.sub.15, L.sub.20, L.sub.25, L.sub.30 is positive at the leftside of the zero point and is negative at the rightside of the zero point. Regarding the voltage, the absolute value of the magnetic strength Hz is detected. Regions 10 mm.ltoreq.X.ltoreq.20 mm, 15 mm.ltoreq.X.ltoreq.25 mm, and 20 mm.ltoreq.X.ltoreq.30 mm are referred to as a segment 2 (S.sub.2), a segment 3 (S.sub.3), and a segment 4 (S.sub.4), respectively.
When the pickup is at the position T (X=21 mm) in FIG. 6, Hz&lt;0 (position T is at the rightside of zero point of the curve L.sub.20) until S.sub.4, X=20 mm, and Hz&gt;0 at S.sub.5, X=25 mm, whereupon the detection voltage V.sub.2 is obtained by the loop L.sub.25. Then the segment two behind segment S5 namely S3 and the loop L.sub.15 are selected, and the detection voltage V.sub.1 thereby is obtained. In this example, S.sub.3 corresponding to the region of 15 mm.ltoreq.X.ltoreq.25 mm is selected as an interpolating segment.
FIG. 7 illustrates an example in which the position of the polarity inversion is shifted. In this example, the distribution curve of the above-mentioned magnetic strength Hz is shifted in the positive direction of X, and like reference numerals designate those similar to the above-mentioned loops and segments. Also in the example, the pickup is disposed in the position T which corresponds to X=21 mm.
In this case, S.sub.4 X=20 mm and hence H.sub.z &gt;0. Therefore, the interpolating region two segment behind is S2, that is, the interpolation is made in the region of 10 mm.ltoreq.X.ltoreq.20 mm.
Thus, because it is out of the region of the original segment, an error would occur in calculating interpolation at that region. In this example, although it is ideal to interpolate at S.sub.3, it must interpolate at S.sub.4 in order to improve the accuracy even if the erroneous discrimination of the segment is made.